1. Field of the Invention
This invention relates to the treatment of effluent, which contains chromophoric compounds and, particularly, aryl azo- and aryl nitro-containing compounds and, optionally, metallized compounds, to reduce the color and the heavy metal content thereof.
2. Description of the Prior Art
Recent concern over the color value of effluents discharged into rivers and streams and enforcement of governmental regulations relating thereto have emphasized the need for effective, yet relatively inexpensive, means for decolorizing such effluent without, in turn, creating other ecological disturbances. One area in which such need exists is in those industries which discharge waste liquor containing ecologically significant amounts of chromophoric compounds such as aryl azo- or aryl nitro-containing compounds. Such effluent is highly colored and may not conform to governmental standards restricting the color value of industrial discharge. Effluent which contains aryl nitro-containing compounds is also objectionable in that these compounds are bactericides and are inimical to anaerobic sewage treatment systems.
In the manufacture of water-soluble dyes, there is always a substantial amount of dye left in solution following "salting" techniques employed for isolating the dyes by filtration. Additionally, there are also many undesired materials such as unreacted starting materials, by-products and decomposition products which are not isolated with the reaction products. Many of these materials are highly colored and also contribute to the color of the effluent.
Usual methods for decolorizing this effluent include oxidation by chlorine, hypochlorites, ozone and the like. These methods are not preferred from an industrial standpoint, however, because they tend to be too costly either because the decolorization is too slow and requires large holding tanks and/or times or because the reactants, themselves, are expensive. Anaerobic systems will destroy a certain amount of color caused by azo dye chromophores but the economics are not attractive.
In Japanese Laid-Open Publication No. 76853/1973 published July 3, 1976, a method is disclosed for decolorizing waste liquor containing azo, phthalocyanine, xanthene and azine dyes. According to the process the liquor is decolorized by adding both (A) a sulfite and/or a bisulfite and (B) a catalytic amount of iron powder and/or ferrosilicon powder to the colored waste liquor. The publication teaches that both (A) and (B) must be present to achieve decolorization. In Reference Example 2 of the publication it is shown that decolorization of a liquor containing a disazo dye (C.I. Direct Blue I) and having an APHA of about 500,000 does not occur with the use of iron powder alone.
The use of sulfite or bisulfite, however, as disclosed in the Japanese Laid-Open Publication is not desired from an industrial standpoint because the sulfite or bisulfite, which is eventually discharged to a sewer, is an extra manufacturing expense and because obnoxious sulfur dioxide is discharged during the treatment.
It is an object of the present invention, therefore, to provide a method for decolorizing effluent containing environmentally significant amounts of color, particularly in the form of aryl azo- and aryl nitro-containing compounds.
It is also an object of the present invention to provide a method for decolorizing effluent containing aryl azo- and aryl nitro-containing compounds which is economically attractive and which avoids the problem of the usual methods employed for decolorization of effluent containing these compounds.
It is a further object of the invention to provide a method for lowering the metal content and the color of effluent containing metallized compounds.
These and other objects of the present invention will become apparent from the following summary and description of preferred embodiments.